manage interpreter instance in interpreter object Currently it is only possible to have one application or interpreter object. Enforce that restriction. * octave.h, octave.cc (application::the_interpreter): Delete. (application::application): Ensure only one application is created at once. * interpreter.cc, interpreter.h (interpreter::instance): New static variable. (interpreter::interpreter): Ensure only one is created at once. (interpreter::~interpreter): Set instance to nullptr. (interpreter::the_interpreter): New function. * interpreter-private.cc, interpreter-private.cc (__get_interpreter__): Delete. Replace all uses with octave::interpreter::the_interpreter.